In reply to a post by prlzx:

The router holds information on hostnames of computers and devices that connect to the network using DHCP.
...snip....

Thank you prizx, that has helped a lot, got thing straight and I seem to be where I want to be now.

I think my first mistake is that both the FritzBox and the NAS had the same name. Let's say 'Woolwich'. I did that for the FritzBox as I also connect and look after another and seeing a different name helps! For the NAS it was an obvious name for the network.

So long story short, I've changed the FritzBox name 'Woolwich-Router' and the NAS to 'Woolwich-Server'. Now my macOS Finder sidebar only lists those two in 'Network'. I can connect as expected to both and no duplicates to confuse me.

Bonus question!

In the Sidebar -> Network, the FritzBox has a blue screen iMac icon and is described as a Mac on the network. The NAS has a server box icon, lots of hard drives and is also a 'Mac'. And when connected/mounted in the sidebar the FritzBox has the same iMac type icon but the NAS has become a generic document, the same icon as 'Documents' uses. Can I get the NAS to have the same iMac icon?

IBM was always and still are more interested in Big business computers
and they do a lot of work for big companies.

They was silly in trusting Microsoft to make windows for them and bill gates stitched them up
after that they wasnt much point IBM being in the PC market as they didnt actually ever make any of the
pc parts they bought them in.

Just the BIOS…. And the book “Accidental Empires” is worth seeking out for a great description of the people whom created all that history back in the early 1980s.